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Title: Thermal induced nano-structural and optical changes of nc-Si:H deposited by hot-wire CVD
Article Type: Original Research - Nano Express
Keywords: Hot-wire CVD; quantum size effects; nano-crystallite; optical band gap
Corresponding Author: Dr Christopher Joseph Arendse, PhD
Corresponding Author's Institution: CSIR National Centre for Nano-Structured Materials, P. O. Box
395, Pretoria 0001, South Africa
First Author: Christopher Joseph Arendse, PhD
Order of Authors: Christopher Joseph Arendse, PhD; Gerald F Malgas, PhD; Theo F Muller, MSc;
Dirk Knoesen, PhD; Clive J Oliphant, MSc; Clive J Oliphant, MSc; David E Motaung, MSc; David E
Motaung, MSc; Gerald F Malgas, PhD; Bonex W Mwakikunga, MSc; Bonex W Mwakikunga, MSc;
Bonex W Mwakikunga, MSc
Manuscript
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1
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4 Thermal induced nano-structural and optical changes of nc-Si:H
5
6 deposited by hot-wire CVD
7
8
9
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11 C. J. Arendse1,, G. F. Malgas1, T. F. G. Muller2, D. Knoesen2, C. J.
12
13 Olpihant1,2, D. E. Motaung1,2 and B. W. Mwakikunga1,3,4
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15
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17 1
18 CSIR National Centre for Nano-Structured Materials, P. O. Box 395, Pretoria
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20 0001, South Africa
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22 2
Department of Physics, University of the Western Cape, Private Bag X17,
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25 Bellville 7535, South Africa
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3
27 School of Physics, University of the Witwatersrand, Private Bag 3, P. O.
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29 Wits, Johannesburg 2050, South Africa
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31 4
32 Department of Physics, University of Malawi, The Polytechnic, Private Bag
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34 303, Blantyre, Malawi
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Abstract
39
40
41 We report on the thermal induced changes of the nano-structural and optical
42
43 properties of hydrogenated nanocrystalline silicon in the temperature range
44
45 200 700 C. The as-deposited sample has a high crystalline volume fraction
46
47
48 of 53% with an average crystallite size of 3.9 nm, where 66% of the total
49
50 hydrogen is bonded as SiH monohydrides on the nano-crystallite surface. A
51
52
53 growth in the native crystallite size and crystalline volume fraction occurs at
54
55 annealing temperatures 400 C, where hydrogen is initially removed from
56
57 the crystallite grain boundaries followed by its removal from the amorphous
58
59
60 Corresponding author: C. J. Arendse (CArendse@csir.co.za)
61 Tel: +27 12 841 3671, Fax: +27 12 841 2229
62
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64 1
65
1
2
3
4 network. The nucleation of smaller nano-crystallites at higher temperatures
5
6 accounts for the enhanced porous structure and the increase in the optical
7
8
9
band gap and average gap.
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11
12
13 Keywords
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15 Hot-wire CVD, quantum size effects, nano-crystallite, optical band gap
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18
19
20 1. Introduction
21
22
23
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25 Hydrogenated nanocrystalline silicon (nc-Si:H) has been the subject of
26
27 intense scientific and technological interest over the past decade, mainly due
28
29 to its reduced photo-induced degradation [1], efficient visible
30
31
photoluminescence [2], tailored optical band gap [3], increased conductivity
32
33
34 and greater doping efficiency [4]. It has been highlighted that these unique
35
36 features are a direct cause of the quantum size effects of the silicon nano-
37
38 crystallites. These improvements make nc-Si:H a potential candidate for
39
40
41 application in photovoltaic and opto-electronic devices [5, 6].
42
43
44
45 The hot-wire chemical vapour deposition (HWCVD) technique, based on the
46
47
catalytic decomposition of the precursor gasses by a heated transition metal
48
49
50 filament, has been established as a viable deposition technique for nc-Si:H
51
52 thin films [6-7]. The structural and opto-electronic properties of the thin films
53
54 are dependent on the deposition parameters, of which the hydrogen dilution
55
56
57 and substrate temperature are the most crucial. It has been established that
58
59 the etching effect of atomic hydrogen, created by the catalytic decomposition
60
61
62
63
64 2
65
1
2
3
4 of H2, is responsible for the termination of weak SiSi bonds from the surface
5
6 and sub-surface regions and that the nucleation of the nano-crystallites are
7
8
9 improved by increasing the hydrogen dilution [7-10]. It has also been reported
10
11 that the hydrogen dilution during deposition determines the concentration and
12
13 the distribution of hydrogen in nc-Si:H, which is closely related to the nano-
14
15
16 structural features; i.e. crystallite size and crystalline volume fraction [11-14].
17
18 These nano-structural features eventually determine the optical properties of
19
20 the material. In particular, the quantum size effects of the Si nano-crystallites
21
22 and the hydrogen concentration have a strong correlation with the optical
23
24
25 band gap [15-16].
26
27
28
29 An investigation into the role of hydrogen in nc-Si:H is therefore crucial for
30
31
32 the understanding of its relation to the nano-structure and the optical
33
34 properties. In this contribution, we investigate the effects of the hydrogen
35
36 concentration and bonding configuration in nc-Si:H deposited by HWCVD on
37
38
the nano-structural features and the optical properties. The hydrogen
39
40
41 concentration and bonding configuration were controlled by post-deposition
42
43 isochronal annealing.
44
45
46
47
48 2. Experimental
49
50
51
52 The nc-Si:H thin film was deposited by the HWCVD process simultaneously
53
54
on single-side polished <100> crystalline silicon and Corning 7059 substrates,
55
56
57 using a mixture of 4 sccm SiH4 and 26 sccm H2 decomposed by seven
58
59 parallel tungsten filaments, 15 cm apart and 36 cm away from the substrates.
60
61
62
63
64 3
65
1
2
3
4 A detailed description of the experimental set-up is given elsewhere [17-18].
5
6 The filament temperature, substrate temperature and deposition pressure
7
8
9 were fixed at 1600 C, 420 C and 60 bar, respectively. The as-deposited
10
11 nc-Si:H thin film was 1140 nm-thick, as measured using a Veeco®
12
13 profilometer.
14
15
16
17
18 Subsequent annealing was performed under high-purity, flowing N2 gas in a
19
20 tube furnace at annealing temperatures (TA) ranging from 200 – 700 C in 100
21
22
23 C increments. The N2 flow rate, heating rate and dwell time for all
24
25 temperatures amounted to 300 sccm, 10C/min and 30 minutes, respectively.
26
27
28 After each annealing temperature, the thin film was allowed to cool to room
29
30 temperature in the tube furnace, while maintaining the N2 flow rate. Thereafter
31
32 the required analytical techniques were performed.
33
34
35
36
37 Fourier transform infrared (FTIR) absorption spectra were collected in
38
39 transmission geometry from 400 – 4000 cm-1 with a spectral resolution of 1
40
41 cm-1, using a Perkin-Elmer Spectrum 100 FTIR spectrophotometer. The
42
43
44 structural properties were investigated using a Jobin-Yvon HR800 micro-
45
46 Raman spectrometer in backscattering geometry at room temperature. The
47
48 Raman spectra were collected in the region 100 – 1000 cm-1 with a spectral
49
50
51
resolution of 0.4 cm-1, using an excitation wavelength of 514.5 nm. X-ray
52
53 diffraction (XRD) spectra were collected in reflection geometry at 2-values
54
55 ranging from 10 – 90 with a step size of 0.02, using a Phillips PW 1830 x-ray
56
57
58 powder diffractometer operating at 45 kV and 40 mA. Copper K1 radiation
59
60 with a wavelength of 1.5406 Å was used as the x-ray source. Optical
61
62
63
64 4
65
1
2
3
4 transmission spectra were measured from 200 – 900 nm with a spectral
5
6 resolution of 1nm, using a Perkin-Elmer LS75S UV/VIS spectrophotometer.
7
8
9
10
11 3. Results and Discussion
12
13
14
15 3.1 Nano-structural properties
16
17
18 FTIR spectroscopy is the established analytical technique of choice to
19
20 probe the silicon-hydrogen bonding configurations and to calculate the
21
22 hydrogen concentration in nc-Si:H and related material. The FTIR absorption
23
24
25 spectrum of the sample in the as-deposited state is shown in Fig. 1. The
26
27 strong absorption bands in the region 920 – 1250 cm-1 is associated with the
28
29 asymmetric SiOSi stretching vibration [19], whereas the peak centred
30
31
32 around 2250 cm-1 is assigned to the HSiO3 vibration [20]. This is indicative
33
34 of an oxidation effect caused by its porous-like microstructure, which is a
35
36 typical feature for nc-Si:H thin films [21-22]. The enhanced absorption band
37
38
39 centred around 640 cm-1 is attributed to the rocking vibrations of all bonding
40
41 configurations of SiHx [23]. The absorption bands in the region 1900 – 2150
42
43 cm-1 is a result of the convolution of several absorption bands associated with
44
45
46 the stretching vibrations of SiHx in different configurations. This is illustrated
47
48 in the insert of Fig. 1, where the absorption spectrum is decomposed into
49
50 three Gaussian components. The absorption peaks centred around 1985 cm-1
51
52
53 and 2090 cm-1 are assigned to the stretching vibrations of SiH
54
55 monohydrides in the amorphous network (isolated) and on the surface of the
56
57 Si nano-crystallites (clustered), respectively [21, 24]. The weak absorption
58
59
60
61
62
63
64 5
65
1
2
3
4 band centred at 2130 cm-1 is assigned to the existence of (SiH2)n
5
6 polyhydride complexes on Si nano-crystallite grain boundaries [25-26].
7
8
9
10
11 To quantify the fraction of H bonded on the surface of nano-crystallites in
12
13 nc-Si:H, we define a structure factor, R s I 2090 / I1985 I 2090 I 2130 , where I
14
15
16 denotes that integrated intensity of each decomposed peak. The total bonded
17
18 hydrogen concentration (CH) was estimated from the integrated absorption of
19
20
the 640 cm-1 rocking mode using previous reported procedures [27-28]. In the
21
22
23 as-deposited state, CH amounts to 2 at.%, characteristic for nc-Si:H
24
25 deposited with high hydrogen dilution [16, 29], where 66% thereof is bonded
26
27
28 on the surface of the nano-crystallites. We propose that this relatively high
29
30 value for Rs is indicative of a high crystalline volume fraction.
31
32
33
34
35
Fig. 2 shows the plots of the hydrogen concentration and the structure factor
36
37 as a function of annealing temperature. The hydrogen concentration and
38
39 structure factor are relatively constant at temperatures below 400 C,
40
41
42
demonstrating that the nano-structure is stable in this temperature regime.
43
44 After annealing at 400 C most of the (SiH2)n polyhydride bonds on the
45
46 grain boundaries of the Si nano-crystallites have been terminated and
47
48
49 consequently results in an increase in the structure factor. Annealing at higher
50
51 temperatures induce a significant decrease in CH, coupled with an increase in
52
53 Rs. These changes are caused by the preferential termination of the isolated
54
55
56
SiH bonds in the amorphous network. We propose that the instability
57
58 induced at TA 400 C is related to the growth of the native nano-crystallites
59
60 and to the nucleation of nano-crystallites in the amorphous network, thereby
61
62
63
64 6
65
1
2
3
4 resulting in an increase in the crystalline volume fraction. It should be noted
5
6 that no SiHx absorption peaks were identified after annealing at 700 C.
7
8
9
10
11 Raman spectroscopy provides direct nano-structural information
12
13 quantitatively related to the average nano-crystallite size and the crystalline
14
15
16 volume fraction in nc-Si:H. Fig. 3 shows the Raman spectra of the sample in
17
18 the as-deposited state and after annealing at specific temperatures. All
19
20 spectra display the following main features: (i) a sharp peak centred around
21
22 515 cm-1, associated with the transverse optic (TO) mode of the nc-Si phase;
23
24
25 (ii) the broad shoulder centred around 480 cm-1, due to the TO-mode of the
26
27 amorphous silicon (a-Si) phase; and (iii) a smaller shoulder around 505 cm-1,
28
29 corresponding to the distribution of crystalline grain boundaries in the sample.
30
31
32
33
34 The crystalline volume fraction, f c [ A505 A515 ]/[ A480 A505 A515 ] , can be
35
36
37
estimated from the integrated areas of the afore-mentioned deconvoluted
38
39 Gaussian peaks [30]. The crystallite size is empirically calculated from
40
41 d Raman 2π B/Δ , where is the shift of the 515 cm-1 peak relative to the
42
43
44 c-Si peak at 520 cm-1 and B 2.0 cm-2 [31]. The quantitative Raman results
45
46 are summarized in Table 1. In the as-deposited state, the average crystallite
47
48
49 size and the crystalline volume fraction amounts to about 3.9 nm and 53%,
50
51 respectively, and remain relatively constant after annealing at 300 C. These
52
53 observations reiterate that the nano-structure of the sample remains stable at
54
55
56 temperatures below 400 C. A blue shift of the nc-Si TO-peak, accompanied
57
58 with a reduction in the intensity of the a-Si TO-peak is observed at annealing
59
60
61
temperatures 400 C, indicative of an increase in the crystallite size and the
62
63
64 7
65
1
2
3
4 crystalline volume fraction, respectively, and supports the claims based on the
5
6 FTIR results.
7
8
9
10
11 XRD was employed as a complimentary method to qualitatively probe the
12
13 changes in the crystallinity as a function of annealing temperature (see Fig.
14
15 4). Three preferential orientations in the <111>, <200> and <311> directions
16
17
18 are observed. The crystallite size in the as-deposited state, estimated from
19
20 the full-width-half-maximum (FWHM) of the (111)-peak, amounts to 19.5 nm
21
22 [32]. A narrowing in the FWHM of the (111)-peak, accompanied with an
23
24
25 increase in its intensity is observed with an increase in annealing temperature.
26
27 This confirms the increase of the crystallite size and crystalline volume
28
29 fraction, as probed by Raman spectroscopy.
30
31
32
33
34 The thermal induced nano-structural changes of the nc-Si:H thin film can be
35
36 interpreted as follows, based on the variation of the SiHx bonding and the
37
38
39 crystalline character. In the as-deposited state the crystalline volume fraction
40
41 is relatively large and therefore the majority of H is bonded to the surface of
42
43 the nano-crystallites. The nano-structural properties are stable at
44
45
46 temperatures below 400 C, attributed to its large crystalline volume fraction.
47
48 An initial increase in the native crystallite size is observed after annealing at
49
50 400 C, resulting in the removal of hydrogen from the grain boundaries. It is
51
52
53 also feasible that smaller crystallites have coalesced into larger crystallites. At
54
55 higher temperatures, hydrogen is removed preferentially from the amorphous
56
57 phase, indicative of the nucleation of smaller nano-crystallites of size 3 nm
58
59
60 in the amorphous network [33], undetected by Raman spectroscopy and XRD.
61
62
63
64 8
65
1
2
3
4 3.2 Optical properties
5
6 The optical properties were determined from UV-visible transmission
7
8
9
measurements, using the method proposed by Swanepoel [34-35]. The
10
11 thickness of the as-deposited sample was calculated to be 1180 nm, which
12
13 concurs to that measured by profilometry. The refractive index n() of a
14
15
16 material is an important optical parameter, since it is directly proportional to
17
18 density [36]. Fig. 5 shows the spectral dependence of the calculated refractive
19
20 index for the sample in the as-deposited state and after annealing at specific
21
22
23 temperatures. A slight increase in the refractive index is observed after
24
25 annealing at 400 C, followed by a decrease at higher temperatures. The
26
27 initial increase can be ascribed to the increase in the native crystallite size
28
29
30 and possibly due to the coalescence of smaller nano-crystallites. Furthermore,
31
32 the presence of (SiH2)n complexes in the as-deposited state is indicative of
33
34 a disordered, porous material and the removal thereof after 400 C would
35
36
37 therefore result in a more compact material. The subsequent decrease of the
38
39 refractive index at higher temperatures is attributed to a more porous
40
41 structure, possibly caused by the nucleation of smaller nano-crystallites in the
42
43
44 amorphous network. Similar trends in the refractive index at zero photon
45
46 energy (no) are observed (see Table 1).
47
48
49
50
51
Detailed analysis of the refractive index spectra were performed using the
52
53 suggested model of Wemple et al [37]. At energies below than of the optical
54
55 band gap, the refractive index is related to the square of the photon energy
56
57
(h)2 by:
58
59
60
61
62
63
64 9
65
1
2
3
4 E ME D
5 n 2 ( ν) 1 (1)
6
E ( hν ) 2
2
M
7
8
9
10 where EM and ED is the average gap and dispersion energy, respectively. The
11
12
13 plot of 1/[n2()-1] versus (h)2 allows for the determination of EM, ED and no.
14
15 The extrapolated results of no and EM, calculated from the linear fit through
16
17 the data, are listed in Table 1.
18
19
20
21
22 The spectral dependence of the absorption coefficient () for the sample in
23
24 the as-deposited state and after annealing at specific temperatures is
25
26
27 depicted in Fig. 6. The optical band gap, referred to as E04, is defined as the
28
29 photon energy where () 104 cm-1, and the values thereof are reported in
30
31
Table 1. A red shift in E04 is observed for TA 600 C followed by an
32
33
34 unexpected blue shift after annealing at 700 C. It is established that the
35
36 optical band gap of hydrogenated amorphous silicon (a-Si:H) deposited by
37
38
39
HWCVD and PECVD increases with an increase in the hydrogen
40
41 concentration [7, 38]. It should be noted that the optical band gap for the
42
43 sample in the as-deposited state is larger than that of a-Si:H with similar CH
44
45 values. This discrepancy is due to the presence of nano-crystallites in the
46
47
48 amorphous network, which lowers the absorption in nc-Si:H and shifts the
49
50 optical band gap towards higher energies [15-16]. The quantum size effect
51
52 size also predicts that an increase in crystallite size is associated with a
53
54
55 decrease in the optical band gap. The initial decrease in E04 after 400 C is
56
57 due to the combined effect of the decreased CH and the growth in the
58
59 crystallite size. After annealing at 600 C the initial hydrogen concentration
60
61
62
63
64 10
65
1
2
3
4 has decreased by 90% with about the same incremental increase in the
5
6
crystallite size as at 400 C, and therefore a more notable decrease in E04 is
7
8
9 expected. However, a minute 0.03 eV decrease in E04 is observed and is
10
11 attributed to the nucleation of smaller nano-crystallites in the amorphous
12
13 network, which explains the competing increasing effect on E04. After
14
15
16 annealing at 700 C, where no hydrogen was detected by FTIR spectroscopy,
17
18 this effect is more pronounced in that an increase in the optical band gap is
19
20 observed.
21
22
23
24
25 The optical band gap and the average gap (EM) have similar behaviours with
26
27 respect to annealing temperature, thereby implying that the growth of the
28
29
30 native nano-crystallites and the nucleation of smaller crystallites in the
31
32 amorphous network have similar effects on the band edges and on the
33
34 conduction and valence bands. Therefore, the average gap can be used to
35
36
describe the thermal induced changes in the optical properties of nc-Si:H.
37
38
39
40
41 4. Conclusion
42
43
44
45
46 The effect of isochronal annealing on the nano-structural and optical
47
48 properties of nc-Si:H, with the emphasis on its relation to the hydrogen
49
50 distribution and concentration, was investigated. Initial changes in the nano-
51
52
53 structure are observed after annealing at 400 C, as evident by termination of
54
55 (SiH2)n polyhydrides from the grain boundaries caused by the growth of the
56
57 native nano-crystallites. At higher temperatures, a further increase in the
58
59
60 native nano-crystallite size and the crystalline volume fraction is observed,
61
62
63
64 11
65
1
2
3
4 accompanied with the nucleation of smaller nano-crystallites and the
5
6 subsequent removal of hydrogen from the amorphous network. At
7
8
9 temperatures 600 C the nucleation of the smaller nano-crystallites results
10
11 in a porous material with an increased optical band gap and average gap,
12
13 explained by the quantum size effect.
14
15
16
17
18 Acknowledgements
19
20 The authors acknowledge the financial assistance of the Department of
21
22 Science and Technology, the National Research Foundation and the Council
23
24
25 for Scientific and Industrial Research (Project no: HGERA2S) of South Africa.
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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Figure 5
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Figure 6
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Table 1
Click here to download Table: Table 1.doc
TA dRaman fc no EM E04
(C) (nm) (%) (eV) (eV)
As-dep. 3.9 53 2.750 3.11 1.88
400 4.7 57 2.758 3.05 1.87
600 5.2 59 2.668 3.03 1.84
700 8.4 64 2.651 3.10 1.87
List of Table and Figure Captions
Click here to download Supplementary Materials: List of Figure and Table Captions.doc
List of Table and Figure Captions
Table 1 Crystallite size, crystalline volume fraction and optical properties
after specific annealing temperatures
Fig. 1 FTIR absorption spectrum of the as-deposited sample and the
deconvolution of the stretching vibrations (insert)
Fig. 2 (a) Hydrogen concentration and (b) the structure factor as a
function of annealing temperature
Fig. 3 Raman spectra of the sample in the as-deposited state and after
annealing at specific temperatures
Fig. 4 XRD spectra of the sample in the as-deposited state and after
annealing at specific temperatures
Fig. 5 Refractive index spectra of the sample in the as-deposited state
and after annealing at specific temperatures
Fig. 6 Absorption coefficient spectra of the sample in the as-deposited
state and after annealing at specific temperatures
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